mojo / core / ports / node.cc [blame]

// Copyright 2016 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifdef UNSAFE_BUFFERS_BUILD
// TODO(crbug.com/351564777): Remove this and convert code to safer constructs.
#pragma allow_unsafe_buffers
#endif

#include "mojo/core/ports/node.h"

#include <string.h>

#include <algorithm>
#include <atomic>
#include <memory>
#include <optional>
#include <utility>
#include <vector>

#include "base/lazy_instance.h"
#include "base/logging.h"
#include "base/memory/ref_counted.h"
#include "base/not_fatal_until.h"
#include "base/notreached.h"
#include "base/rand_util.h"
#include "base/synchronization/lock.h"
#include "base/threading/thread_local.h"
#include "build/build_config.h"
#include "mojo/core/ports/event.h"
#include "mojo/core/ports/node_delegate.h"
#include "mojo/core/ports/port_locker.h"
#include "third_party/abseil-cpp/absl/container/inlined_vector.h"

namespace mojo {
namespace core {
namespace ports {

namespace {

constexpr size_t kRandomNameCacheSize = 256;

// Random port name generator which maintains a cache of random bytes to draw
// from. This amortizes the cost of random name generation on platforms where
// RandBytes may have significant per-call overhead.
//
// Note that the use of this cache means one has to be careful about fork()ing
// a process once any port names have been generated, as that behavior can lead
// to collisions between independently generated names in different processes.
class RandomNameGenerator {
 public:
  RandomNameGenerator() = default;

  RandomNameGenerator(const RandomNameGenerator&) = delete;
  RandomNameGenerator& operator=(const RandomNameGenerator&) = delete;

  ~RandomNameGenerator() = default;

  PortName GenerateRandomPortName() {
    base::AutoLock lock(lock_);
    if (cache_index_ == kRandomNameCacheSize) {
      base::RandBytes(base::as_writable_byte_span(cache_));
      cache_index_ = 0;
    }
    return cache_[cache_index_++];
  }

 private:
  base::Lock lock_;
  PortName cache_[kRandomNameCacheSize];
  size_t cache_index_ = kRandomNameCacheSize;
};

base::LazyInstance<RandomNameGenerator>::Leaky g_name_generator =
    LAZY_INSTANCE_INITIALIZER;

int DebugError(const char* message, int error_code) {
  NOTREACHED() << "Oops: " << message;
}

#define OOPS(x) DebugError(#x, x)

bool CanAcceptMoreMessages(const Port* port) {
  // Have we already doled out the last message (i.e., do we expect to NOT
  // receive further messages)?
  uint64_t next_sequence_num = port->message_queue.next_sequence_num();
  if (port->state == Port::kClosed)
    return false;
  if (port->peer_closed || port->remove_proxy_on_last_message) {
    if (port->peer_lost_unexpectedly)
      return port->message_queue.HasNextMessage();
    if (port->last_sequence_num_to_receive == next_sequence_num - 1)
      return false;
  }
  return true;
}

void GenerateRandomPortName(PortName* name) {
  *name = g_name_generator.Get().GenerateRandomPortName();
}

}  // namespace

Node::Node(const NodeName& name, NodeDelegate* delegate)
    : name_(name), delegate_(this, delegate) {}

Node::~Node() {
  if (!ports_.empty())
    DLOG(WARNING) << "Unclean shutdown for node " << name_;
}

bool Node::CanShutdownCleanly(ShutdownPolicy policy) {
  PortLocker::AssertNoPortsLockedOnCurrentThread();
  base::AutoLock ports_lock(ports_lock_);

  if (policy == ShutdownPolicy::DONT_ALLOW_LOCAL_PORTS) {
#if DCHECK_IS_ON()
    for (auto& entry : ports_) {
      DVLOG(2) << "Port " << entry.first << " referencing node "
               << entry.second->peer_node_name << " is blocking shutdown of "
               << "node " << name_ << " (state=" << entry.second->state << ")";
    }
#endif
    return ports_.empty();
  }

  DCHECK_EQ(policy, ShutdownPolicy::ALLOW_LOCAL_PORTS);

  // NOTE: This is not efficient, though it probably doesn't need to be since
  // relatively few ports should be open during shutdown and shutdown doesn't
  // need to be blazingly fast.
  bool can_shutdown = true;
  for (auto& entry : ports_) {
    PortRef port_ref(entry.first, entry.second);
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->peer_node_name != name_ && port->state != Port::kReceiving) {
      can_shutdown = false;
#if DCHECK_IS_ON()
      DVLOG(2) << "Port " << entry.first << " referencing node "
               << port->peer_node_name << " is blocking shutdown of "
               << "node " << name_ << " (state=" << port->state << ")";
#else
      // Exit early when not debugging.
      break;
#endif
    }
  }

  return can_shutdown;
}

int Node::GetPort(const PortName& port_name, PortRef* port_ref) {
  PortLocker::AssertNoPortsLockedOnCurrentThread();
  base::AutoLock lock(ports_lock_);
  auto iter = ports_.find(port_name);
  if (iter == ports_.end())
    return ERROR_PORT_UNKNOWN;

#if BUILDFLAG(IS_ANDROID) && defined(ARCH_CPU_ARM64)
  // Workaround for https://crbug.com/665869.
  std::atomic_thread_fence(std::memory_order_seq_cst);
#endif

  *port_ref = PortRef(port_name, iter->second);
  return OK;
}

int Node::CreateUninitializedPort(PortRef* port_ref) {
  PortName port_name;
  GenerateRandomPortName(&port_name);

  scoped_refptr<Port> port(new Port(kInitialSequenceNum, kInitialSequenceNum));
  int rv = AddPortWithName(port_name, port);
  if (rv != OK)
    return rv;

  *port_ref = PortRef(port_name, std::move(port));
  return OK;
}

int Node::InitializePort(const PortRef& port_ref,
                         const NodeName& peer_node_name,
                         const PortName& peer_port_name,
                         const NodeName& prev_node_name,
                         const PortName& prev_port_name) {
  {
    // Must be acquired for UpdatePortPeerAddress below.
    PortLocker::AssertNoPortsLockedOnCurrentThread();
    base::AutoLock ports_locker(ports_lock_);

    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state != Port::kUninitialized)
      return ERROR_PORT_STATE_UNEXPECTED;

    port->state = Port::kReceiving;
    UpdatePortPeerAddress(port_ref.name(), port, peer_node_name,
                          peer_port_name);

    port->prev_node_name = prev_node_name;
    port->prev_port_name = prev_port_name;
  }

  delegate_->PortStatusChanged(port_ref);

  return OK;
}

int Node::CreatePortPair(PortRef* port0_ref, PortRef* port1_ref) {
  int rv;

  rv = CreateUninitializedPort(port0_ref);
  if (rv != OK)
    return rv;

  rv = CreateUninitializedPort(port1_ref);
  if (rv != OK)
    return rv;

  rv = InitializePort(*port0_ref, name_, port1_ref->name(), name_,
                      port1_ref->name());
  if (rv != OK)
    return rv;

  rv = InitializePort(*port1_ref, name_, port0_ref->name(), name_,
                      port0_ref->name());
  if (rv != OK)
    return rv;

  return OK;
}

int Node::SetUserData(const PortRef& port_ref,
                      scoped_refptr<UserData> user_data) {
  SinglePortLocker locker(&port_ref);
  auto* port = locker.port();
  if (port->state == Port::kClosed)
    return ERROR_PORT_STATE_UNEXPECTED;

  port->user_data = std::move(user_data);

  return OK;
}

int Node::GetUserData(const PortRef& port_ref,
                      scoped_refptr<UserData>* user_data) {
  SinglePortLocker locker(&port_ref);
  auto* port = locker.port();
  if (port->state == Port::kClosed)
    return ERROR_PORT_STATE_UNEXPECTED;

  *user_data = port->user_data;

  return OK;
}

int Node::ClosePort(const PortRef& port_ref) {
  std::vector<std::unique_ptr<UserMessageEvent>> undelivered_messages;
  NodeName peer_node_name;
  PortName peer_port_name;
  uint64_t sequence_num = 0;
  uint64_t last_sequence_num = 0;
  bool was_initialized = false;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    switch (port->state) {
      case Port::kUninitialized:
        break;

      case Port::kReceiving:
        was_initialized = true;
        port->state = Port::kClosed;

        // We pass along the sequence number of the last message sent from this
        // port to allow the peer to have the opportunity to consume all inbound
        // messages before notifying the embedder that this port is closed.
        last_sequence_num = port->next_sequence_num_to_send - 1;

        peer_node_name = port->peer_node_name;
        peer_port_name = port->peer_port_name;

        sequence_num = port->next_control_sequence_num_to_send++;

        // If the port being closed still has unread messages, then we need to
        // take care to close those ports so as to avoid leaking memory.
        port->message_queue.TakeAllMessages(&undelivered_messages);
        port->TakePendingMessages(undelivered_messages);
        break;

      default:
        return ERROR_PORT_STATE_UNEXPECTED;
    }
  }

  ErasePort(port_ref.name());

  if (was_initialized) {
    DVLOG(2) << "Sending ObserveClosure from " << port_ref.name() << "@"
             << name_ << " to " << peer_port_name << "@" << peer_node_name;
    delegate_->ForwardEvent(
        peer_node_name,
        std::make_unique<ObserveClosureEvent>(peer_port_name, port_ref.name(),
                                              sequence_num, last_sequence_num));
    for (const auto& message : undelivered_messages) {
      for (size_t i = 0; i < message->num_ports(); ++i) {
        PortRef ref;
        if (GetPort(message->ports()[i], &ref) == OK)
          ClosePort(ref);
      }
    }
  }
  return OK;
}

int Node::GetStatus(const PortRef& port_ref, PortStatus* port_status) {
  SinglePortLocker locker(&port_ref);
  auto* port = locker.port();
  if (port->state != Port::kReceiving)
    return ERROR_PORT_STATE_UNEXPECTED;

  // TODO(sroettger): include messages pending sender verification here?
  port_status->has_messages = port->message_queue.HasNextMessage();
  port_status->receiving_messages = CanAcceptMoreMessages(port);
  port_status->peer_closed = port->peer_closed;
  port_status->peer_remote = port->peer_node_name != name_;
  port_status->queued_message_count =
      port->message_queue.queued_message_count();
  port_status->queued_num_bytes = port->message_queue.queued_num_bytes();
  port_status->unacknowledged_message_count =
      port->next_sequence_num_to_send - port->last_sequence_num_acknowledged -
      1;

  return OK;
}

int Node::GetMessage(const PortRef& port_ref,
                     std::unique_ptr<UserMessageEvent>* message,
                     MessageFilter* filter) {
  *message = nullptr;

  DVLOG(4) << "GetMessage for " << port_ref.name() << "@" << name_;

  NodeName peer_node_name;
  ScopedEvent ack_event;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    // This could also be treated like the port being unknown since the
    // embedder should no longer be referring to a port that has been sent.
    if (port->state != Port::kReceiving)
      return ERROR_PORT_STATE_UNEXPECTED;

    // Let the embedder get messages until there are no more before reporting
    // that the peer closed its end.
    if (!CanAcceptMoreMessages(port))
      return ERROR_PORT_PEER_CLOSED;

    port->message_queue.GetNextMessage(message, filter);
    if (*message &&
        (*message)->sequence_num() == port->sequence_num_to_acknowledge) {
      peer_node_name = port->peer_node_name;
      ack_event = std::make_unique<UserMessageReadAckEvent>(
          port->peer_port_name, port_ref.name(),
          port->next_control_sequence_num_to_send++,
          port->sequence_num_to_acknowledge);
    }
    if (*message) {
      // Message will be passed to the user, no need to block the queue.
      port->message_queue.MessageProcessed();
    }
  }

  if (ack_event)
    delegate_->ForwardEvent(peer_node_name, std::move(ack_event));

  // Allow referenced ports to trigger PortStatusChanged calls.
  if (*message) {
    for (size_t i = 0; i < (*message)->num_ports(); ++i) {
      PortRef new_port_ref;
      int rv = GetPort((*message)->ports()[i], &new_port_ref);

      DCHECK_EQ(OK, rv) << "Port " << new_port_ref.name() << "@" << name_
                        << " does not exist!";

      SinglePortLocker locker(&new_port_ref);
      DCHECK_EQ(locker.port()->state, Port::kReceiving);
      locker.port()->message_queue.set_signalable(true);
    }

    // The user may retransmit this message from another port. We reset the
    // sequence number so that the message will get a new one if that happens.
    (*message)->set_sequence_num(0);
  }

  return OK;
}

int Node::SendUserMessage(const PortRef& port_ref,
                          std::unique_ptr<UserMessageEvent> message) {
  int rv = SendUserMessageInternal(port_ref, &message);
  if (rv != OK) {
    // If send failed, close all carried ports. Note that we're careful not to
    // close the sending port itself if it happened to be one of the encoded
    // ports (an invalid but possible condition.)
    for (size_t i = 0; i < message->num_ports(); ++i) {
      if (message->ports()[i] == port_ref.name())
        continue;

      PortRef port;
      if (GetPort(message->ports()[i], &port) == OK)
        ClosePort(port);
    }
  }
  return rv;
}

int Node::SetAcknowledgeRequestInterval(
    const PortRef& port_ref,
    uint64_t sequence_num_acknowledge_interval) {
  NodeName peer_node_name;
  PortName peer_port_name;
  uint64_t sequence_num_to_request_ack = 0;
  uint64_t sequence_num = 0;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state != Port::kReceiving)
      return ERROR_PORT_STATE_UNEXPECTED;

    port->sequence_num_acknowledge_interval = sequence_num_acknowledge_interval;
    if (!sequence_num_acknowledge_interval)
      return OK;

    peer_node_name = port->peer_node_name;
    peer_port_name = port->peer_port_name;

    sequence_num_to_request_ack = port->last_sequence_num_acknowledged +
                                  sequence_num_acknowledge_interval;
    sequence_num = port->next_control_sequence_num_to_send++;
  }

  delegate_->ForwardEvent(peer_node_name,
                          std::make_unique<UserMessageReadAckRequestEvent>(
                              peer_port_name, port_ref.name(), sequence_num,
                              sequence_num_to_request_ack));
  return OK;
}

bool Node::IsEventFromPreviousPeer(const Event& event) {
  switch (event.type()) {
    case Event::Type::kUserMessage:
      return true;
    case Event::Type::kPortAccepted:
      // PortAccepted is sent by the next peer
      return false;
    case Event::Type::kObserveProxy:
      // ObserveProxy with an invalid port name is a broadcast event
      return event.port_name() != kInvalidPortName;
    case Event::Type::kObserveProxyAck:
      return true;
    case Event::Type::kObserveClosure:
      return true;
    case Event::Type::kMergePort:
      // MergePort is not from the previous peer
      return false;
    case Event::Type::kUserMessageReadAckRequest:
      return true;
    case Event::Type::kUserMessageReadAck:
      return true;
    case Event::Type::kUpdatePreviousPeer:
      return true;
    default:
      // No need to check unknown message types since AcceptPeer will return
      // an error.
      return false;
  }
}

int Node::AcceptEventInternal(const PortRef& port_ref,
                              const NodeName& from_node,
                              ScopedEvent event) {
  switch (event->type()) {
    case Event::Type::kUserMessage:
      return OnUserMessage(port_ref, from_node,
                           Event::Cast<UserMessageEvent>(&event));
    case Event::Type::kPortAccepted:
      return OnPortAccepted(port_ref, Event::Cast<PortAcceptedEvent>(&event));
    case Event::Type::kObserveProxy:
      return OnObserveProxy(port_ref, Event::Cast<ObserveProxyEvent>(&event));
    case Event::Type::kObserveProxyAck:
      return OnObserveProxyAck(port_ref,
                               Event::Cast<ObserveProxyAckEvent>(&event));
    case Event::Type::kObserveClosure:
      return OnObserveClosure(port_ref,
                              Event::Cast<ObserveClosureEvent>(&event));
    case Event::Type::kMergePort:
      return OnMergePort(port_ref, Event::Cast<MergePortEvent>(&event));
    case Event::Type::kUserMessageReadAckRequest:
      return OnUserMessageReadAckRequest(
          port_ref, Event::Cast<UserMessageReadAckRequestEvent>(&event));
    case Event::Type::kUserMessageReadAck:
      return OnUserMessageReadAck(port_ref,
                                  Event::Cast<UserMessageReadAckEvent>(&event));
    case Event::Type::kUpdatePreviousPeer:
      return OnUpdatePreviousPeer(port_ref,
                                  Event::Cast<UpdatePreviousPeerEvent>(&event));
  }
  return OOPS(ERROR_NOT_IMPLEMENTED);
}

int Node::AcceptEvent(const NodeName& from_node, ScopedEvent event) {
  PortRef port_ref;
  GetPort(event->port_name(), &port_ref);

#ifndef MOJO_BACKWARDS_COMPAT
  DVLOG(2) << "AcceptEvent type: " << event->type() << ", "
           << event->from_port() << "@" << from_node << " => "
           << port_ref.name() << "@" << name_
           << " seq nr: " << event->control_sequence_num() << " port valid? "
           << port_ref.is_valid();

  if (!IsEventFromPreviousPeer(*event)) {
    DCHECK_EQ(event->control_sequence_num(), kInvalidSequenceNum);
    // Some events are not coming from the previous peer, e.g. broadcasts or
    // PortAccepted events. No need to check the sequence number or sender.
    return AcceptEventInternal(port_ref, from_node, std::move(event));
  }

  DCHECK_NE(event->control_sequence_num(), kInvalidSequenceNum);

  if (!port_ref.is_valid()) {
    // If we don't have a valid port, there's nothing for us to check. However,
    // we pass the ref on to AcceptEventInternal to make sure there's no race
    // where it becomes valid and we skipped the peer check.
    return AcceptEventInternal(port_ref, from_node, std::move(event));
  }

  // Before processing the event, verify the sender and sequence number.
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (!port->IsNextEvent(from_node, *event)) {
      DVLOG(2) << "Buffering event (type " << event->type()
               << "): " << event->from_port() << "@" << from_node << " => "
               << port_ref.name() << "@" << name_
               << " seq nr: " << event->control_sequence_num() << " / "
               << port->next_control_sequence_num_to_receive << ", want "
               << port->prev_port_name << "@" << port->prev_node_name;

      port->BufferEvent(from_node, std::move(event));
      return OK;
    }
  }

  int ret = AcceptEventInternal(port_ref, from_node, std::move(event));

  // More events might have been enqueued during processing.
  while (true) {
    ScopedEvent next_event;
    NodeName next_from_node;
    {
      SinglePortLocker locker(&port_ref);
      auto* port = locker.port();
      // We always increment the control sequence number after we finished
      // processing the event. That way we ensure that the events are handled
      // in order without keeping a lock the whole time.
      port->next_control_sequence_num_to_receive++;
      port->NextEvent(&next_from_node, &next_event);

      if (next_event) {
        DVLOG(2) << "Handling buffered event (type " << next_event->type()
                 << "): " << next_event->from_port() << "@" << next_from_node
                 << " => " << port_ref.name() << "@" << name_
                 << " seq nr: " << next_event->control_sequence_num() << " / "
                 << port->next_control_sequence_num_to_receive;
      }
    }
    if (!next_event)
      break;
    AcceptEventInternal(port_ref, next_from_node, std::move(next_event));
  }

  return ret;
#else
  return AcceptEventInternal(port_ref, from_node, std::move(event));
#endif
}

int Node::MergePorts(const PortRef& port_ref,
                     const NodeName& destination_node_name,
                     const PortName& destination_port_name) {
  PortName new_port_name;
  Event::PortDescriptor new_port_descriptor;
  PendingUpdatePreviousPeer pending_update_event{.from_port = port_ref.name()};
  {
    // Must be held for ConvertToProxy.
    PortLocker::AssertNoPortsLockedOnCurrentThread();
    base::AutoLock ports_locker(ports_lock_);

    SinglePortLocker locker(&port_ref);

    DVLOG(1) << "Sending MergePort from " << port_ref.name() << "@" << name_
             << " to " << destination_port_name << "@" << destination_node_name;

    // Send the port-to-merge over to the destination node so it can be merged
    // into the port cycle atomically there.
    new_port_name = port_ref.name();
    ConvertToProxy(locker.port(), destination_node_name, &new_port_name,
                   &new_port_descriptor, &pending_update_event);
  }

#ifndef MOJO_BACKWARDS_COMPAT
  delegate_->ForwardEvent(
      pending_update_event.receiver,
      std::make_unique<UpdatePreviousPeerEvent>(
          pending_update_event.port, pending_update_event.from_port,
          pending_update_event.sequence_num, pending_update_event.new_prev_node,
          pending_update_event.new_prev_port));
#endif

  if (new_port_descriptor.peer_node_name == name_ &&
      destination_node_name != name_) {
    // Ensure that the locally retained peer of the new proxy gets a status
    // update so it notices that its peer is now remote.
    PortRef local_peer;
    if (GetPort(new_port_descriptor.peer_port_name, &local_peer) == OK)
      delegate_->PortStatusChanged(local_peer);
  }

  delegate_->ForwardEvent(
      destination_node_name,
      std::make_unique<MergePortEvent>(destination_port_name, kInvalidPortName,
                                       kInvalidSequenceNum, new_port_name,
                                       new_port_descriptor));
  return OK;
}

int Node::MergeLocalPorts(const PortRef& port0_ref, const PortRef& port1_ref) {
  DVLOG(1) << "Merging local ports " << port0_ref.name() << "@" << name_
           << " and " << port1_ref.name() << "@" << name_;
  return MergePortsInternal(port0_ref, port1_ref,
                            true /* allow_close_on_bad_state */);
}

int Node::LostConnectionToNode(const NodeName& node_name) {
  // We can no longer send events to the given node. We also can't expect any
  // PortAccepted events.

  DVLOG(1) << "Observing lost connection from node " << name_ << " to node "
           << node_name;

  DestroyAllPortsWithPeer(node_name, kInvalidPortName);
  return OK;
}

int Node::OnUserMessage(const PortRef& port_ref,
                        const NodeName& from_node,
                        std::unique_ptr<UserMessageEvent> message) {
#if DCHECK_IS_ON()
  std::ostringstream ports_buf;
  for (size_t i = 0; i < message->num_ports(); ++i) {
    if (i > 0)
      ports_buf << ",";
    ports_buf << message->ports()[i];
  }

  DVLOG(4) << "OnUserMessage " << message->sequence_num()
           << " [ports=" << ports_buf.str() << "] at " << message->port_name()
           << "@" << name_;
#endif

  // Even if this port does not exist, cannot receive anymore messages or is
  // buffering or proxying messages, we still need these ports to be bound to
  // this node. When the message is forwarded, these ports will get transferred
  // following the usual method. If the message cannot be accepted, then the
  // newly bound ports will simply be closed.
  if (from_node != name_) {
    for (size_t i = 0; i < message->num_ports(); ++i) {
      Event::PortDescriptor& descriptor = message->port_descriptors()[i];
      int rv = AcceptPort(message->ports()[i], descriptor);
      if (rv != OK)
        return rv;
    }
  }

  bool has_next_message = false;
  bool message_accepted = false;
  bool should_forward_messages = false;
  if (port_ref.is_valid()) {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    // Reject spurious messages if we've already received the last expected
    // message.
    if (CanAcceptMoreMessages(port)) {
      message_accepted = true;
      port->message_queue.AcceptMessage(std::move(message), &has_next_message);

      if (port->state == Port::kBuffering) {
        has_next_message = false;
      } else if (port->state == Port::kProxying) {
        has_next_message = false;
        should_forward_messages = true;
      }
    }
  }

  if (should_forward_messages) {
    int rv = ForwardUserMessagesFromProxy(port_ref);
    if (rv != OK)
      return rv;
    TryRemoveProxy(port_ref);
  }

  if (!message_accepted) {
    DVLOG(2) << "Message not accepted!\n";
    // Close all newly accepted ports as they are effectively orphaned.
    for (size_t i = 0; i < message->num_ports(); ++i) {
      PortRef attached_port_ref;
      if (GetPort(message->ports()[i], &attached_port_ref) == OK) {
        ClosePort(attached_port_ref);
      } else {
        DLOG(WARNING) << "Cannot close non-existent port!\n";
      }
    }
  } else if (has_next_message) {
    delegate_->PortStatusChanged(port_ref);
  }

  return OK;
}

int Node::OnPortAccepted(const PortRef& port_ref,
                         std::unique_ptr<PortAcceptedEvent> event) {
  if (!port_ref.is_valid())
    return ERROR_PORT_UNKNOWN;

#if DCHECK_IS_ON()
  {
    SinglePortLocker locker(&port_ref);
    DVLOG(2) << "PortAccepted at " << port_ref.name() << "@" << name_
             << " pointing to " << locker.port()->peer_port_name << "@"
             << locker.port()->peer_node_name;
  }
#endif

  return BeginProxying(port_ref);
}

int Node::OnObserveProxy(const PortRef& port_ref,
                         std::unique_ptr<ObserveProxyEvent> event) {
  if (event->port_name() == kInvalidPortName) {
    // An ObserveProxy with an invalid target port name is a broadcast used to
    // inform ports when their peer (which was itself a proxy) has become
    // defunct due to unexpected node disconnection.
    //
    // Receiving ports affected by this treat it as equivalent to peer closure.
    // Proxies affected by this can be removed and will in turn broadcast their
    // own death with a similar message.
    DCHECK_EQ(event->proxy_target_node_name(), kInvalidNodeName);
    DCHECK_EQ(event->proxy_target_port_name(), kInvalidPortName);
    DestroyAllPortsWithPeer(event->proxy_node_name(), event->proxy_port_name());
    return OK;
  }

  // The port may have already been closed locally, in which case the
  // ObserveClosure message will contain the last_sequence_num field.
  // We can then silently ignore this message.
  if (!port_ref.is_valid()) {
    DVLOG(1) << "ObserveProxy: " << event->port_name() << "@" << name_
             << " not found";
    return OK;
  }

  DVLOG(2) << "ObserveProxy at " << port_ref.name() << "@" << name_
           << ", proxy at " << event->proxy_port_name() << "@"
           << event->proxy_node_name() << " pointing to "
           << event->proxy_target_port_name() << "@"
           << event->proxy_target_node_name();

  bool peer_changed = false;
  ScopedEvent event_to_forward;
  NodeName event_target_node;
  {
    // Must be acquired for UpdatePortPeerAddress below.
    PortLocker::AssertNoPortsLockedOnCurrentThread();
    base::AutoLock ports_locker(ports_lock_);

    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    if (port->peer_node_name == event->proxy_node_name() &&
        port->peer_port_name == event->proxy_port_name()) {
      if (port->state == Port::kReceiving) {
        // Updating the port peer will reset the sequence num. Grab it now;
        uint64_t sequence_num = port->next_control_sequence_num_to_send++;
        UpdatePortPeerAddress(port_ref.name(), port,
                              event->proxy_target_node_name(),
                              event->proxy_target_port_name());
        event_target_node = event->proxy_node_name();
        event_to_forward = std::make_unique<ObserveProxyAckEvent>(
            event->proxy_port_name(), port_ref.name(), sequence_num,
            port->next_sequence_num_to_send - 1);
        peer_changed = true;
        DVLOG(2) << "Forwarding ObserveProxyAck from " << event->port_name()
                 << "@" << name_ << " to " << event->proxy_port_name() << "@"
                 << event_target_node;
      } else {
        // As a proxy ourselves, we don't know how to honor the ObserveProxy
        // event or to populate the last_sequence_num field of ObserveProxyAck.
        // Afterall, another port could be sending messages to our peer now
        // that we've sent out our own ObserveProxy event.  Instead, we will
        // send an ObserveProxyAck indicating that the ObserveProxy event
        // should be re-sent (last_sequence_num set to kInvalidSequenceNum).
        // However, this has to be done after we are removed as a proxy.
        // Otherwise, we might just find ourselves back here again, which
        // would be akin to a busy loop.

        DVLOG(2) << "Delaying ObserveProxyAck to " << event->proxy_port_name()
                 << "@" << event->proxy_node_name();

        port->send_on_proxy_removal =
            std::make_unique<std::pair<NodeName, ScopedEvent>>(
                event->proxy_node_name(),
                std::make_unique<ObserveProxyAckEvent>(
                    event->proxy_port_name(), port_ref.name(),
                    kInvalidSequenceNum, kInvalidSequenceNum));
      }
    } else {
      // Forward this event along to our peer. Eventually, it should find the
      // port referring to the proxy.
      event_target_node = port->peer_node_name;
      event->set_port_name(port->peer_port_name);
      event->set_from_port(port_ref.name());
      event->set_control_sequence_num(
          port->next_control_sequence_num_to_send++);
      if (port->state == Port::kBuffering) {
        port->control_message_queue.push({event_target_node, std::move(event)});
      } else {
        event_to_forward = std::move(event);
      }
    }
  }

  if (event_to_forward)
    delegate_->ForwardEvent(event_target_node, std::move(event_to_forward));

  if (peer_changed) {
    // Re-send ack and/or ack requests, as the previous peer proxy may not have
    // forwarded the previous request before it died.
    MaybeResendAck(port_ref);
    MaybeResendAckRequest(port_ref);

    delegate_->PortStatusChanged(port_ref);
  }

  return OK;
}

int Node::OnObserveProxyAck(const PortRef& port_ref,
                            std::unique_ptr<ObserveProxyAckEvent> event) {
  DVLOG(2) << "ObserveProxyAck at " << event->port_name() << "@" << name_
           << " (last_sequence_num=" << event->last_sequence_num() << ")";

  if (!port_ref.is_valid())
    return ERROR_PORT_UNKNOWN;  // The port may have observed closure first.

  bool try_remove_proxy_immediately;
  bool erase_port = false;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    if (port->state == Port::kProxying) {
      // If the last sequence number is invalid, this is a signal that we need
      // to retransmit the ObserveProxy event for this port rather than flagging
      // the the proxy for removal ASAP.
      try_remove_proxy_immediately =
          event->last_sequence_num() != kInvalidSequenceNum;
      if (try_remove_proxy_immediately) {
        // We can now remove this port once we have received and forwarded the
        // last message addressed to this port.
        port->remove_proxy_on_last_message = true;
        port->last_sequence_num_to_receive = event->last_sequence_num();
      }
    } else if (port->state == Port::kClosed) {
      erase_port = true;
    } else {
      return OOPS(ERROR_PORT_STATE_UNEXPECTED);
    }
  }

  if (erase_port) {
    ErasePort(port_ref.name());
    return OK;
  }

  if (try_remove_proxy_immediately)
    TryRemoveProxy(port_ref);
  else
    InitiateProxyRemoval(port_ref);

  return OK;
}

int Node::OnObserveClosure(const PortRef& port_ref,
                           std::unique_ptr<ObserveClosureEvent> event) {
  // OK if the port doesn't exist, as it may have been closed already.
  if (!port_ref.is_valid())
    return OK;

  // This message tells the port that it should no longer expect more messages
  // beyond last_sequence_num. This message is forwarded along until we reach
  // the receiving end, and this message serves as an equivalent to
  // ObserveProxyAck.

  bool notify_delegate = false;
  NodeName peer_node_name;
  bool try_remove_proxy = false;
  bool erase_port = false;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    port->peer_closed = true;
    port->last_sequence_num_to_receive = event->last_sequence_num();

    DVLOG(2) << "ObserveClosure at " << port_ref.name() << "@" << name_
             << " (state=" << port->state << ") pointing to "
             << port->peer_port_name << "@" << port->peer_node_name
             << " (last_sequence_num=" << event->last_sequence_num() << ")";

    // We always forward ObserveClosure, even beyond the receiving port which
    // cares about it. This ensures that any dead-end proxies beyond that port
    // are notified to remove themselves.

    if (port->state == Port::kReceiving) {
      notify_delegate = true;

      // When forwarding along the other half of the port cycle, this will only
      // reach dead-end proxies. Tell them we've sent our last message so they
      // can go away.
      //
      // TODO: Repurposing ObserveClosure for this has the desired result but
      // may be semantically confusing since the forwarding port is not actually
      // closed. Consider replacing this with a new event type.
      event->set_last_sequence_num(port->next_sequence_num_to_send - 1);

      // Treat the closure as an acknowledge that all sent messages have been
      // read from the other end.
      port->last_sequence_num_acknowledged =
          port->next_sequence_num_to_send - 1;
    } else if (port->state == Port::kClosed) {
      // This is the ack for a closed proxy port notification. Now it's fine to
      // delete the port.
      erase_port = true;
    } else {
      // We haven't yet reached the receiving peer of the closed port, so we'll
      // forward the message along as-is.
      // See about removing the port if it is a proxy as our peer won't be able
      // to participate in proxy removal.
      port->remove_proxy_on_last_message = true;
      if (port->state == Port::kProxying)
        try_remove_proxy = true;
    }

    DVLOG(2) << "Forwarding ObserveClosure from " << port_ref.name() << "@"
             << name_ << " to peer " << port->peer_port_name << "@"
             << port->peer_node_name
             << " (last_sequence_num=" << event->last_sequence_num() << ")";

    event->set_port_name(port->peer_port_name);
    event->set_from_port(port_ref.name());
    event->set_control_sequence_num(port->next_control_sequence_num_to_send++);
    peer_node_name = port->peer_node_name;

    if (port->state == Port::kBuffering) {
      port->control_message_queue.push({peer_node_name, std::move(event)});
    }
  }

  if (try_remove_proxy)
    TryRemoveProxy(port_ref);

  if (erase_port)
    ErasePort(port_ref.name());

  if (event)
    delegate_->ForwardEvent(peer_node_name, std::move(event));

  if (notify_delegate)
    delegate_->PortStatusChanged(port_ref);

  return OK;
}

int Node::OnMergePort(const PortRef& port_ref,
                      std::unique_ptr<MergePortEvent> event) {
  DVLOG(1) << "MergePort at " << port_ref.name() << "@" << name_
           << " merging with proxy " << event->new_port_name() << "@" << name_
           << " pointing to " << event->new_port_descriptor().peer_port_name
           << "@" << event->new_port_descriptor().peer_node_name
           << " referred by "
           << event->new_port_descriptor().referring_port_name << "@"
           << event->new_port_descriptor().referring_node_name;

  // Accept the new port. This is now the receiving end of the other port cycle
  // to be merged with ours. Note that we always attempt to accept the new port
  // first as otherwise its peer receiving port could be left stranded
  // indefinitely.
  if (AcceptPort(event->new_port_name(), event->new_port_descriptor()) != OK) {
    if (port_ref.is_valid())
      ClosePort(port_ref);
    return ERROR_PORT_STATE_UNEXPECTED;
  }

  PortRef new_port_ref;
  GetPort(event->new_port_name(), &new_port_ref);
  if (!port_ref.is_valid() && new_port_ref.is_valid()) {
    ClosePort(new_port_ref);
    return ERROR_PORT_UNKNOWN;
  } else if (port_ref.is_valid() && !new_port_ref.is_valid()) {
    ClosePort(port_ref);
    return ERROR_PORT_UNKNOWN;
  }

  bool peer_allowed = true;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (!port->pending_merge_peer) {
      LOG(ERROR) << "MergePort called on unexpected port: "
                 << event->port_name();
      peer_allowed = false;
    } else {
      port->pending_merge_peer = false;
    }
  }
  if (!peer_allowed) {
    ClosePort(port_ref);
    return ERROR_PORT_STATE_UNEXPECTED;
  }

  return MergePortsInternal(port_ref, new_port_ref,
                            false /* allow_close_on_bad_state */);
}

int Node::OnUserMessageReadAckRequest(
    const PortRef& port_ref,
    std::unique_ptr<UserMessageReadAckRequestEvent> event) {
  DVLOG(1) << "AckRequest " << port_ref.name() << "@" << name_ << " sequence "
           << event->sequence_num_to_acknowledge();

  if (!port_ref.is_valid())
    return ERROR_PORT_UNKNOWN;

  NodeName peer_node_name;
  std::unique_ptr<Event> event_to_send;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    peer_node_name = port->peer_node_name;
    if (port->state == Port::kProxying) {
      // Proxies simply forward the ack request to their peer.
      event->set_port_name(port->peer_port_name);
      event->set_from_port(port_ref.name());
      event->set_control_sequence_num(
          port->next_control_sequence_num_to_send++);
      event_to_send = std::move(event);
    } else {
      uint64_t current_sequence_num =
          port->message_queue.next_sequence_num() - 1;
      // Either this is requesting an ack for a sequence number already read, or
      // else for a sequence number that is yet to be read.
      if (current_sequence_num >= event->sequence_num_to_acknowledge()) {
        // If the current sequence number to read already exceeds the ack
        // request, send an ack immediately.
        event_to_send = std::make_unique<UserMessageReadAckEvent>(
            port->peer_port_name, port_ref.name(),
            port->next_control_sequence_num_to_send++, current_sequence_num);

        if (port->state == Port::kBuffering) {
          port->control_message_queue.push(
              {peer_node_name, std::move(event_to_send)});
        }

        // This might be a late or duplicate acknowledge request, that's
        // requesting acknowledge for an already read message. There may already
        // have been a request for future reads, so take care not to back up
        // the requested acknowledge counter.
        if (current_sequence_num > port->sequence_num_to_acknowledge)
          port->sequence_num_to_acknowledge = current_sequence_num;
      } else {
        // This is request to ack a sequence number that hasn't been read yet.
        // The state of the port can either be that it already has a
        // future-requested ack, or not. Because ack requests aren't guaranteed
        // to arrive in order, store the earlier of the current  queued request
        // and the new one, if one was already requested.
        bool has_queued_ack_request =
            port->sequence_num_to_acknowledge > current_sequence_num;
        if (!has_queued_ack_request ||
            port->sequence_num_to_acknowledge >
                event->sequence_num_to_acknowledge()) {
          port->sequence_num_to_acknowledge =
              event->sequence_num_to_acknowledge();
        }
        return OK;
      }
    }
  }

  if (event_to_send)
    delegate_->ForwardEvent(peer_node_name, std::move(event_to_send));

  return OK;
}

int Node::OnUserMessageReadAck(const PortRef& port_ref,
                               std::unique_ptr<UserMessageReadAckEvent> event) {
  DVLOG(1) << "Acknowledge " << port_ref.name() << "@" << name_ << " sequence "
           << event->sequence_num_acknowledged();

  NodeName peer_node_name;
  ScopedEvent ack_request_event;
  if (port_ref.is_valid()) {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    if (event->sequence_num_acknowledged() >= port->next_sequence_num_to_send) {
      // TODO(http://crbug.com/980952): This is a malformed event.
      //      This could return a new error "ERROR_MALFORMED_EVENT" which the
      //      delegate could use as a signal to drop the peer node.
      return OK;
    }

    // Keep the largest acknowledge seen.
    if (event->sequence_num_acknowledged() <=
        port->last_sequence_num_acknowledged) {
      // The acknowledge was late or a duplicate, it's safe to ignore it.
      return OK;
    }

    port->last_sequence_num_acknowledged = event->sequence_num_acknowledged();
    // Send another ack request if the interval is non-zero and the peer has
    // not been closed.
    if (port->sequence_num_acknowledge_interval && !port->peer_closed) {
      peer_node_name = port->peer_node_name;
      ack_request_event = std::make_unique<UserMessageReadAckRequestEvent>(
          port->peer_port_name, port_ref.name(),
          port->next_control_sequence_num_to_send++,
          port->last_sequence_num_acknowledged +
              port->sequence_num_acknowledge_interval);
      DCHECK_NE(port->state, Port::kBuffering);
    }
  }
  if (ack_request_event)
    delegate_->ForwardEvent(peer_node_name, std::move(ack_request_event));

  if (port_ref.is_valid())
    delegate_->PortStatusChanged(port_ref);

  return OK;
}

int Node::OnUpdatePreviousPeer(const PortRef& port_ref,
                               std::unique_ptr<UpdatePreviousPeerEvent> event) {
  DVLOG(1) << "OnUpdatePreviousPeer port: " << event->port_name()
           << " changing to " << event->new_node_name()
           << ", port: " << event->from_port() << " => "
           << event->new_port_name();

  if (!port_ref.is_valid()) {
    return ERROR_PORT_UNKNOWN;
  }

  const NodeName& new_node_name = event->new_node_name();
  const PortName& new_port_name = event->new_port_name();
  DCHECK_NE(new_node_name, kInvalidNodeName);
  DCHECK_NE(new_port_name, kInvalidPortName);
  if (new_node_name == kInvalidNodeName || new_port_name == kInvalidPortName) {
    return ERROR_PORT_STATE_UNEXPECTED;
  }

  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();

    port->prev_node_name = new_node_name;
    port->prev_port_name = new_port_name;
    // The sequence number will get incremented after this event has been
    // handled.
    port->next_control_sequence_num_to_receive = kInitialSequenceNum - 1;
  }

  return OK;
}

int Node::AddPortWithName(const PortName& port_name, scoped_refptr<Port> port) {
  PortLocker::AssertNoPortsLockedOnCurrentThread();
  base::AutoLock lock(ports_lock_);
  if (port->peer_port_name != kInvalidPortName) {
    DCHECK_NE(kInvalidNodeName, port->peer_node_name);
    peer_port_maps_[port->peer_node_name][port->peer_port_name].emplace(
        port_name, PortRef(port_name, port));
  }
  if (!ports_.emplace(port_name, std::move(port)).second)
    return OOPS(ERROR_PORT_EXISTS);  // Suggests a bad UUID generator.
  DVLOG(2) << "Created port " << port_name << "@" << name_;
  return OK;
}

void Node::ErasePort(const PortName& port_name) {
  PortLocker::AssertNoPortsLockedOnCurrentThread();
  scoped_refptr<Port> port;
  {
    base::AutoLock lock(ports_lock_);
    auto it = ports_.find(port_name);
    if (it == ports_.end())
      return;
    port = std::move(it->second);
    ports_.erase(it);

    RemoveFromPeerPortMap(port_name, port.get());
  }
  // NOTE: We are careful not to release the port's messages while holding any
  // locks, since they may run arbitrary user code upon destruction.
  std::vector<std::unique_ptr<UserMessageEvent>> messages;
  {
    PortRef port_ref(port_name, std::move(port));
    SinglePortLocker locker(&port_ref);
    locker.port()->message_queue.TakeAllMessages(&messages);
  }
  DVLOG(2) << "Deleted port " << port_name << "@" << name_;
}

int Node::SendUserMessageInternal(const PortRef& port_ref,
                                  std::unique_ptr<UserMessageEvent>* message) {
  std::unique_ptr<UserMessageEvent>& m = *message;

  m->set_from_port(port_ref.name());

  for (size_t i = 0; i < m->num_ports(); ++i) {
    if (m->ports()[i] == port_ref.name())
      return ERROR_PORT_CANNOT_SEND_SELF;
  }

  NodeName target_node;
  int rv = PrepareToForwardUserMessage(port_ref, Port::kReceiving,
                                       false /* ignore_closed_peer */, m.get(),
                                       &target_node);
  if (rv != OK)
    return rv;

  // Beyond this point there's no sense in returning anything but OK. Even if
  // message forwarding or acceptance fails, there's nothing the embedder can
  // do to recover. Assume that failure beyond this point must be treated as a
  // transport failure.

  DCHECK_NE(kInvalidNodeName, target_node);
  if (target_node != name_) {
    delegate_->ForwardEvent(target_node, std::move(m));
    return OK;
  }

  int accept_result = AcceptEvent(name_, std::move(m));
  if (accept_result != OK) {
    // See comment above for why we don't return an error in this case.
    DVLOG(2) << "AcceptEvent failed: " << accept_result;
  }

  return OK;
}

int Node::MergePortsInternal(const PortRef& port0_ref,
                             const PortRef& port1_ref,
                             bool allow_close_on_bad_state) {
  const PortRef* port_refs[2] = {&port0_ref, &port1_ref};
  PendingUpdatePreviousPeer pending_update_events[2];
  uint64_t original_sequence_number[2];
  {
    // Needed to swap peer map entries below.
    PortLocker::AssertNoPortsLockedOnCurrentThread();
    base::ReleasableAutoLock ports_locker(&ports_lock_);

    std::optional<PortLocker> locker(std::in_place, port_refs, 2);
    auto* port0 = locker->GetPort(port0_ref);
    auto* port1 = locker->GetPort(port1_ref);

    // There are several conditions which must be met before we'll consider
    // merging two ports:
    //
    // - They must both be in the kReceiving state
    // - They must not be each other's peer
    // - They must have never sent a user message
    //
    // If any of these criteria are not met, we fail early.
    if (port0->state != Port::kReceiving || port1->state != Port::kReceiving ||
        (port0->peer_node_name == name_ &&
         port0->peer_port_name == port1_ref.name()) ||
        (port1->peer_node_name == name_ &&
         port1->peer_port_name == port0_ref.name()) ||
        port0->next_sequence_num_to_send != kInitialSequenceNum ||
        port1->next_sequence_num_to_send != kInitialSequenceNum) {
      // On failure, we only close a port if it was at least properly in the
      // |kReceiving| state. This avoids getting the system in an inconsistent
      // state by e.g. closing a proxy abruptly.
      //
      // Note that we must release the port locks before closing ports.
      const bool close_port0 =
          port0->state == Port::kReceiving || allow_close_on_bad_state;
      const bool close_port1 =
          port1->state == Port::kReceiving || allow_close_on_bad_state;
      locker.reset();
      ports_locker.Release();
      if (close_port0)
        ClosePort(port0_ref);
      if (close_port1)
        ClosePort(port1_ref);
      return ERROR_PORT_STATE_UNEXPECTED;
    }

    pending_update_events[0] = {
        .receiver = port0->peer_node_name,
        .port = port0->peer_port_name,
        .from_port = port0_ref.name(),
        .sequence_num = port0->next_control_sequence_num_to_send++,
        .new_prev_node = name_,
        .new_prev_port = port1_ref.name()};
    pending_update_events[1] = {
        .receiver = port1->peer_node_name,
        .port = port1->peer_port_name,
        .from_port = port1_ref.name(),
        .sequence_num = port1->next_control_sequence_num_to_send++,
        .new_prev_node = name_,
        .new_prev_port = port0_ref.name()};

    // Swap the ports' peer information and switch them both to proxying mode.
    SwapPortPeers(port0_ref.name(), port0, port1_ref.name(), port1);
    port0->state = Port::kProxying;
    port1->state = Port::kProxying;
    original_sequence_number[0] = port0->next_control_sequence_num_to_send;
    original_sequence_number[1] = port1->next_control_sequence_num_to_send;
    port0->next_control_sequence_num_to_send = kInitialSequenceNum;
    port1->next_control_sequence_num_to_send = kInitialSequenceNum;
    if (port0->peer_closed)
      port0->remove_proxy_on_last_message = true;
    if (port1->peer_closed)
      port1->remove_proxy_on_last_message = true;
  }

  // Flush any queued messages from the new proxies and, if successful, complete
  // the merge by initiating proxy removals.
  if (ForwardUserMessagesFromProxy(port0_ref) == OK &&
      ForwardUserMessagesFromProxy(port1_ref) == OK) {
#ifndef MOJO_BACKWARDS_COMPAT
    // Send the prev peer updates out after the forwarding the user messages
    // succeeded. Otherwise, we won't be able to restore the previous state
    // below.
    for (const auto& pending_update_event : pending_update_events) {
      delegate_->ForwardEvent(
          pending_update_event.receiver,
          std::make_unique<UpdatePreviousPeerEvent>(
              pending_update_event.port, pending_update_event.from_port,
              pending_update_event.sequence_num,
              pending_update_event.new_prev_node,
              pending_update_event.new_prev_port));
    }
#endif

    for (const auto* const port_ref : port_refs) {
      bool try_remove_proxy_immediately = false;
      ScopedEvent closure_event;
      NodeName closure_event_target_node;
      {
        SinglePortLocker locker(port_ref);
        auto* port = locker.port();
        DCHECK_EQ(port->state, Port::kProxying);
        try_remove_proxy_immediately = port->remove_proxy_on_last_message;
        if (try_remove_proxy_immediately || port->peer_closed) {
          // If either end of the port cycle is closed, we propagate an
          // ObserveClosure event.
          closure_event_target_node = port->peer_node_name;
          closure_event = std::make_unique<ObserveClosureEvent>(
              port->peer_port_name, port_ref->name(),
              port->next_control_sequence_num_to_send++,
              port->last_sequence_num_to_receive);
        }
      }
      if (try_remove_proxy_immediately)
        TryRemoveProxy(*port_ref);
      else
        InitiateProxyRemoval(*port_ref);

      if (closure_event) {
        delegate_->ForwardEvent(closure_event_target_node,
                                std::move(closure_event));
      }
    }

    return OK;
  }

  // If we failed to forward proxied messages, we keep the system in a
  // consistent state by undoing the peer swap and closing the ports.
  {
    PortLocker::AssertNoPortsLockedOnCurrentThread();
    base::AutoLock ports_locker(ports_lock_);
    PortLocker locker(port_refs, 2);
    auto* port0 = locker.GetPort(port0_ref);
    auto* port1 = locker.GetPort(port1_ref);
    SwapPortPeers(port0_ref.name(), port0, port1_ref.name(), port1);
    port0->remove_proxy_on_last_message = false;
    port1->remove_proxy_on_last_message = false;
    DCHECK_EQ(Port::kProxying, port0->state);
    DCHECK_EQ(Port::kProxying, port1->state);
    port0->state = Port::kReceiving;
    port1->state = Port::kReceiving;
    port0->next_control_sequence_num_to_send = original_sequence_number[0];
    port1->next_control_sequence_num_to_send = original_sequence_number[1];
  }

  ClosePort(port0_ref);
  ClosePort(port1_ref);
  return ERROR_PORT_STATE_UNEXPECTED;
}

void Node::ConvertToProxy(Port* port,
                          const NodeName& to_node_name,
                          PortName* port_name,
                          Event::PortDescriptor* port_descriptor,
                          PendingUpdatePreviousPeer* pending_update) {
  port->AssertLockAcquired();
  PortName local_port_name = *port_name;

  PortName new_port_name;
  GenerateRandomPortName(&new_port_name);

  pending_update->receiver = port->peer_node_name;
  pending_update->port = port->peer_port_name;
  pending_update->sequence_num = port->next_control_sequence_num_to_send++;
  pending_update->new_prev_node = to_node_name;
  pending_update->new_prev_port = new_port_name;

  // Make sure we don't send messages to the new peer until after we know it
  // exists. In the meantime, just buffer messages locally.
  DCHECK_EQ(port->state, Port::kReceiving);
  port->state = Port::kBuffering;

  // If we already know our peer is closed, we already know this proxy can
  // be removed once it receives and forwards its last expected message.
  if (port->peer_closed)
    port->remove_proxy_on_last_message = true;

  *port_name = new_port_name;

  port_descriptor->peer_node_name = port->peer_node_name;
  port_descriptor->peer_port_name = port->peer_port_name;
  port_descriptor->referring_node_name = name_;
  port_descriptor->referring_port_name = local_port_name;
  port_descriptor->next_sequence_num_to_send = port->next_sequence_num_to_send;
  port_descriptor->next_sequence_num_to_receive =
      port->message_queue.next_sequence_num();
  port_descriptor->last_sequence_num_to_receive =
      port->last_sequence_num_to_receive;
  port_descriptor->peer_closed = port->peer_closed;
  memset(port_descriptor->padding, 0, sizeof(port_descriptor->padding));

  // Configure the local port to point to the new port.
  UpdatePortPeerAddress(local_port_name, port, to_node_name, new_port_name);
}

int Node::AcceptPort(const PortName& port_name,
                     const Event::PortDescriptor& port_descriptor) {
  scoped_refptr<Port> port =
      base::MakeRefCounted<Port>(port_descriptor.next_sequence_num_to_send,
                                 port_descriptor.next_sequence_num_to_receive);
  port->state = Port::kReceiving;
  port->peer_node_name = port_descriptor.peer_node_name;
  port->peer_port_name = port_descriptor.peer_port_name;
  port->next_control_sequence_num_to_send = kInitialSequenceNum;
  port->next_control_sequence_num_to_receive = kInitialSequenceNum;
  port->prev_node_name = port_descriptor.referring_node_name;
  port->prev_port_name = port_descriptor.referring_port_name;
  port->last_sequence_num_to_receive =
      port_descriptor.last_sequence_num_to_receive;
  port->peer_closed = port_descriptor.peer_closed;

  DVLOG(2) << "Accepting port " << port_name
           << " [peer_closed=" << port->peer_closed
           << "; last_sequence_num_to_receive="
           << port->last_sequence_num_to_receive << "]";

  // A newly accepted port is not signalable until the message referencing the
  // new port finds its way to the consumer (see GetMessage).
  port->message_queue.set_signalable(false);

  int rv = AddPortWithName(port_name, std::move(port));
  if (rv != OK)
    return rv;

  // Allow referring port to forward messages.
  delegate_->ForwardEvent(port_descriptor.referring_node_name,
                          std::make_unique<PortAcceptedEvent>(
                              port_descriptor.referring_port_name,
                              kInvalidPortName, kInvalidSequenceNum));
  return OK;
}

int Node::PrepareToForwardUserMessage(const PortRef& forwarding_port_ref,
                                      Port::State expected_port_state,
                                      bool ignore_closed_peer,
                                      UserMessageEvent* message,
                                      NodeName* forward_to_node) {
  bool target_is_remote = false;
  base::queue<PendingUpdatePreviousPeer> peer_update_events;

  for (;;) {
    NodeName target_node_name;
    {
      SinglePortLocker locker(&forwarding_port_ref);
      target_node_name = locker.port()->peer_node_name;
    }

    // NOTE: This may call out to arbitrary user code, so it's important to call
    // it only while no port locks are held on the calling thread.
    if (target_node_name != name_) {
      if (!message->NotifyWillBeRoutedExternally()) {
        LOG(ERROR) << "NotifyWillBeRoutedExternally failed unexpectedly.";
        return ERROR_PORT_STATE_UNEXPECTED;
      }
    }

    // Must be held because ConvertToProxy needs to update |peer_port_maps_|.
    PortLocker::AssertNoPortsLockedOnCurrentThread();
    base::AutoLock ports_locker(ports_lock_);

    // Simultaneously lock the forwarding port as well as all attached ports.
    absl::InlinedVector<PortRef, 4> attached_port_refs;
    absl::InlinedVector<const PortRef*, 5> ports_to_lock;
    attached_port_refs.resize(message->num_ports());
    ports_to_lock.resize(message->num_ports() + 1);
    ports_to_lock[0] = &forwarding_port_ref;
    for (size_t i = 0; i < message->num_ports(); ++i) {
      const PortName& attached_port_name = message->ports()[i];
      auto iter = ports_.find(attached_port_name);
      CHECK(iter != ports_.end(), base::NotFatalUntil::M130);
      attached_port_refs[i] = PortRef(attached_port_name, iter->second);
      ports_to_lock[i + 1] = &attached_port_refs[i];
    }
    PortLocker locker(ports_to_lock.data(), ports_to_lock.size());
    auto* forwarding_port = locker.GetPort(forwarding_port_ref);

    if (forwarding_port->peer_node_name != target_node_name) {
      // The target node has already changed since we last held the lock.
      if (target_node_name == name_) {
        // If the target node was previously this local node, we need to restart
        // the loop, since that means we may now route the message externally.
        continue;
      }

      target_node_name = forwarding_port->peer_node_name;
    }
    target_is_remote = target_node_name != name_;

    if (forwarding_port->state != expected_port_state)
      return ERROR_PORT_STATE_UNEXPECTED;
    if (forwarding_port->peer_closed && !ignore_closed_peer)
      return ERROR_PORT_PEER_CLOSED;

    // Messages may already have a sequence number if they're being forwarded by
    // a proxy. Otherwise, use the next outgoing sequence number.
    if (message->sequence_num() == 0)
      message->set_sequence_num(forwarding_port->next_sequence_num_to_send++);
#if DCHECK_IS_ON()
    std::ostringstream ports_buf;
    for (size_t i = 0; i < message->num_ports(); ++i) {
      if (i > 0)
        ports_buf << ",";
      ports_buf << message->ports()[i];
    }
#endif

    if (message->num_ports() > 0) {
      // Sanity check to make sure we can actually send all the attached ports.
      // They must all be in the |kReceiving| state and must not be the sender's
      // own peer.
      DCHECK_EQ(message->num_ports(), attached_port_refs.size());
      for (size_t i = 0; i < message->num_ports(); ++i) {
        auto* attached_port = locker.GetPort(attached_port_refs[i]);
        int error = OK;
        if (attached_port->state != Port::kReceiving) {
          error = ERROR_PORT_STATE_UNEXPECTED;
        } else if (attached_port_refs[i].name() ==
                   forwarding_port->peer_port_name) {
          error = ERROR_PORT_CANNOT_SEND_PEER;
        }

        if (error != OK) {
          // Not going to send. Backpedal on the sequence number.
          forwarding_port->next_sequence_num_to_send--;
          return error;
        }
      }

      if (target_is_remote) {
        // We only bother to proxy and rewrite ports in the event if it's
        // going to be routed to an external node. This substantially reduces
        // the amount of port churn in the system, as many port-carrying
        // events are routed at least 1 or 2 intra-node hops before (if ever)
        // being routed externally.
        Event::PortDescriptor* port_descriptors = message->port_descriptors();
        for (size_t i = 0; i < message->num_ports(); ++i) {
          auto* port = locker.GetPort(attached_port_refs[i]);
          PendingUpdatePreviousPeer update_event = {
              .from_port = attached_port_refs[i].name()};
          ConvertToProxy(port, target_node_name, message->ports() + i,
                         port_descriptors + i, &update_event);
          peer_update_events.push(update_event);
        }
      }
    }

#if DCHECK_IS_ON()
    DVLOG(4) << "Sending message " << message->sequence_num()
             << " [ports=" << ports_buf.str() << "]"
             << " from " << forwarding_port_ref.name() << "@" << name_ << " to "
             << forwarding_port->peer_port_name << "@" << target_node_name;
#endif

    *forward_to_node = target_node_name;
    message->set_port_name(forwarding_port->peer_port_name);
    message->set_from_port(forwarding_port_ref.name());
    message->set_control_sequence_num(
        forwarding_port->next_control_sequence_num_to_send++);
    break;
  }

#ifndef MOJO_BACKWARDS_COMPAT
  while (!peer_update_events.empty()) {
    auto pending_update_event = peer_update_events.front();
    peer_update_events.pop();
    delegate_->ForwardEvent(
        pending_update_event.receiver,
        std::make_unique<UpdatePreviousPeerEvent>(
            pending_update_event.port, pending_update_event.from_port,
            pending_update_event.sequence_num,
            pending_update_event.new_prev_node,
            pending_update_event.new_prev_port));
  }
#endif

  if (target_is_remote) {
    for (size_t i = 0; i < message->num_ports(); ++i) {
      // For any ports that were converted to proxies above, make sure their
      // prior local peer (if applicable) receives a status update so it can be
      // made aware of its peer's location.
      const Event::PortDescriptor& descriptor = message->port_descriptors()[i];
      if (descriptor.peer_node_name == name_) {
        PortRef local_peer;
        if (GetPort(descriptor.peer_port_name, &local_peer) == OK)
          delegate_->PortStatusChanged(local_peer);
      }
    }
  }

  return OK;
}

int Node::BeginProxying(const PortRef& port_ref) {
  base::queue<std::pair<NodeName, ScopedEvent>> control_message_queue;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state != Port::kBuffering)
      return OOPS(ERROR_PORT_STATE_UNEXPECTED);
    port->state = Port::kProxying;
    std::swap(port->control_message_queue, control_message_queue);
  }

  while (!control_message_queue.empty()) {
    auto node_event_pair = std::move(control_message_queue.front());
    control_message_queue.pop();
    delegate_->ForwardEvent(node_event_pair.first,
                            std::move(node_event_pair.second));
  }

  int rv = ForwardUserMessagesFromProxy(port_ref);
  if (rv != OK)
    return rv;

  // Forward any pending acknowledge request.
  MaybeForwardAckRequest(port_ref);

  bool try_remove_proxy_immediately;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state != Port::kProxying)
      return OOPS(ERROR_PORT_STATE_UNEXPECTED);

    try_remove_proxy_immediately = port->remove_proxy_on_last_message;
  }

  if (try_remove_proxy_immediately) {
    TryRemoveProxy(port_ref);
  } else {
    InitiateProxyRemoval(port_ref);
  }

  return OK;
}

int Node::ForwardUserMessagesFromProxy(const PortRef& port_ref) {
  for (;;) {
    // NOTE: We forward messages in sequential order here so that we maintain
    // the message queue's notion of next sequence number. That's useful for the
    // proxy removal process as we can tell when this port has seen all of the
    // messages it is expected to see.
    std::unique_ptr<UserMessageEvent> message;
    {
      SinglePortLocker locker(&port_ref);
      locker.port()->message_queue.GetNextMessage(&message, nullptr);
      if (!message)
        break;
    }

    NodeName target_node;
    int rv = PrepareToForwardUserMessage(port_ref, Port::kProxying,
                                         true /* ignore_closed_peer */,
                                         message.get(), &target_node);
    {
      // Mark the message as processed after we ran PrepareToForwardUserMessage.
      // This is important to prevent another thread from deleting the port
      // before we grabbed a sequence number for the message.
      SinglePortLocker locker(&port_ref);
      locker.port()->message_queue.MessageProcessed();
    }
    if (rv != OK)
      return rv;

    delegate_->ForwardEvent(target_node, std::move(message));
  }
  return OK;
}

void Node::InitiateProxyRemoval(const PortRef& port_ref) {
  NodeName peer_node_name;
  PortName peer_port_name;
  uint64_t sequence_num;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state == Port::kClosed)
      return;
    peer_node_name = port->peer_node_name;
    peer_port_name = port->peer_port_name;
    sequence_num = port->next_control_sequence_num_to_send++;
    DCHECK_EQ(port->state, Port::kProxying);
  }

  // To remove this node, we start by notifying the connected graph that we are
  // a proxy. This allows whatever port is referencing this node to skip it.
  // Eventually, this node will receive ObserveProxyAck (or ObserveClosure if
  // the peer was closed in the meantime).
  delegate_->ForwardEvent(
      peer_node_name, std::make_unique<ObserveProxyEvent>(
                          peer_port_name, port_ref.name(), sequence_num, name_,
                          port_ref.name(), peer_node_name, peer_port_name));
}

void Node::TryRemoveProxy(const PortRef& port_ref) {
  bool should_erase = false;
  NodeName removal_target_node;
  ScopedEvent removal_event;
  PendingUpdatePreviousPeer pending_update_event;

  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state == Port::kClosed)
      return;
    DCHECK_EQ(port->state, Port::kProxying);

    // Make sure we have seen ObserveProxyAck before removing the port.
    if (!port->remove_proxy_on_last_message)
      return;

    if (!CanAcceptMoreMessages(port)) {
      DCHECK_EQ(port->message_queue.queued_message_count(), 0lu);
      should_erase = true;
      if (port->send_on_proxy_removal) {
        removal_target_node = port->send_on_proxy_removal->first;
        removal_event = std::move(port->send_on_proxy_removal->second);
        if (removal_event) {
          removal_event->set_control_sequence_num(
              port->next_control_sequence_num_to_send++);
          DCHECK_EQ(removal_target_node, port->peer_node_name);
          DCHECK_EQ(removal_event->port_name(), port->peer_port_name);
        }
      }
      // Tell the peer_node to accept messages from prev_node from now.
      pending_update_event = {
          .receiver = port->peer_node_name,
          .port = port->peer_port_name,
          .from_port = port_ref.name(),
          .sequence_num = port->next_control_sequence_num_to_send++,
          .new_prev_node = port->prev_node_name,
          .new_prev_port = port->prev_port_name};
    } else {
      DVLOG(2) << "Cannot remove port " << port_ref.name() << "@" << name_
               << " now; waiting for more messages";
    }
  }

  if (should_erase) {
#ifndef MOJO_BACKWARDS_COMPAT
    delegate_->ForwardEvent(
        pending_update_event.receiver,
        std::make_unique<UpdatePreviousPeerEvent>(
            pending_update_event.port, pending_update_event.from_port,
            pending_update_event.sequence_num,
            pending_update_event.new_prev_node,
            pending_update_event.new_prev_port));
#endif
    ErasePort(port_ref.name());
  }

  if (removal_event)
    delegate_->ForwardEvent(removal_target_node, std::move(removal_event));
}

void Node::DestroyAllPortsWithPeer(const NodeName& node_name,
                                   const PortName& port_name) {
  // Wipes out all ports whose peer node matches |node_name| and whose peer port
  // matches |port_name|. If |port_name| is |kInvalidPortName|, only the peer
  // node is matched.

  std::vector<PortRef> ports_to_notify;
  std::vector<PortName> dead_proxies_to_broadcast;
  std::vector<std::unique_ptr<UserMessageEvent>> undelivered_messages;

  ScopedEvent closure_event;
  NodeName closure_event_target_node;

  {
    PortLocker::AssertNoPortsLockedOnCurrentThread();
    base::AutoLock ports_lock(ports_lock_);

    auto node_peer_port_map_iter = peer_port_maps_.find(node_name);
    if (node_peer_port_map_iter == peer_port_maps_.end())
      return;

    auto& node_peer_port_map = node_peer_port_map_iter->second;
    auto peer_ports_begin = node_peer_port_map.begin();
    auto peer_ports_end = node_peer_port_map.end();
    if (port_name != kInvalidPortName) {
      // If |port_name| is given, we limit the set of local ports to the ones
      // with that specific port as their peer.
      peer_ports_begin = node_peer_port_map.find(port_name);
      if (peer_ports_begin == node_peer_port_map.end())
        return;

      peer_ports_end = peer_ports_begin;
      ++peer_ports_end;
    }

    for (auto peer_port_iter = peer_ports_begin;
         peer_port_iter != peer_ports_end; ++peer_port_iter) {
      auto& local_ports = peer_port_iter->second;
      // NOTE: This inner loop almost always has only one element. There are
      // relatively short-lived cases where more than one local port points to
      // the same peer, and this only happens when extra ports are bypassed
      // proxies waiting to be torn down.
      for (auto local_port_iter = local_ports.begin();
           local_port_iter != local_ports.end(); ++local_port_iter) {
        auto& local_port_ref = local_port_iter->second;

        SinglePortLocker locker(&local_port_ref);
        auto* port = locker.port();

        if (port_name != kInvalidPortName) {
          // If this is a targeted observe dead proxy event, send out an
          // ObserveClosure to acknowledge it.
          closure_event_target_node = port->peer_node_name;
          closure_event = std::make_unique<ObserveClosureEvent>(
              port->peer_port_name, local_port_ref.name(),
              port->next_control_sequence_num_to_send++,
              port->last_sequence_num_to_receive);
        }

        if (!port->peer_closed) {
          // Treat this as immediate peer closure. It's an exceptional
          // condition akin to a broken pipe, so we don't care about losing
          // messages.

          port->peer_closed = true;
          port->peer_lost_unexpectedly = true;
          if (port->state == Port::kReceiving)
            ports_to_notify.push_back(local_port_ref);
        }

        // We don't expect to forward any further messages, and we don't
        // expect to receive a Port{Accepted,Rejected} event. Because we're
        // a proxy with no active peer, we cannot use the normal proxy removal
        // procedure of forward-propagating an ObserveProxy. Instead we
        // broadcast our own death so it can be back-propagated. This is
        // inefficient but rare.
        if (port->state == Port::kBuffering || port->state == Port::kProxying) {
          port->state = Port::kClosed;
          dead_proxies_to_broadcast.push_back(local_port_ref.name());
          std::vector<std::unique_ptr<UserMessageEvent>> messages;
          port->message_queue.TakeAllMessages(&messages);
          port->TakePendingMessages(messages);
          for (auto& message : messages)
            undelivered_messages.emplace_back(std::move(message));
        }
      }
    }
  }

#ifdef MOJO_BACKWARDS_COMPAT
  for (const auto& proxy_name : dead_proxies_to_broadcast) {
    ErasePort(proxy_name);
    DVLOG(2) << "Forcibly deleted port " << proxy_name << "@" << name_;
  }
#endif

  if (closure_event) {
    delegate_->ForwardEvent(closure_event_target_node,
                            std::move(closure_event));
  }

  // Wake up any receiving ports who have just observed simulated peer closure.
  for (const auto& port : ports_to_notify)
    delegate_->PortStatusChanged(port);

  for (const auto& proxy_name : dead_proxies_to_broadcast) {
    // Broadcast an event signifying that this proxy is no longer functioning.
    delegate_->BroadcastEvent(std::make_unique<ObserveProxyEvent>(
        kInvalidPortName, kInvalidPortName, kInvalidSequenceNum, name_,
        proxy_name, kInvalidNodeName, kInvalidPortName));

    // Also process death locally since the port that points this closed one
    // could be on the current node.
    // Note: Although this is recursive, only a single port is involved which
    // limits the expected branching to 1.
    DestroyAllPortsWithPeer(name_, proxy_name);
  }

  // Close any ports referenced by undelivered messages.
  for (const auto& message : undelivered_messages) {
    for (size_t i = 0; i < message->num_ports(); ++i) {
      PortRef ref;
      if (GetPort(message->ports()[i], &ref) == OK)
        ClosePort(ref);
    }
  }
}

void Node::UpdatePortPeerAddress(const PortName& local_port_name,
                                 Port* local_port,
                                 const NodeName& new_peer_node,
                                 const PortName& new_peer_port) {
  ports_lock_.AssertAcquired();
  local_port->AssertLockAcquired();

  RemoveFromPeerPortMap(local_port_name, local_port);
  local_port->peer_node_name = new_peer_node;
  local_port->peer_port_name = new_peer_port;
  local_port->next_control_sequence_num_to_send = kInitialSequenceNum;
  if (new_peer_port != kInvalidPortName) {
    peer_port_maps_[new_peer_node][new_peer_port].emplace(
        local_port_name,
        PortRef(local_port_name, base::WrapRefCounted<Port>(local_port)));
  }
}

void Node::RemoveFromPeerPortMap(const PortName& local_port_name,
                                 Port* local_port) {
  if (local_port->peer_port_name == kInvalidPortName)
    return;

  auto node_iter = peer_port_maps_.find(local_port->peer_node_name);
  if (node_iter == peer_port_maps_.end())
    return;

  auto& node_peer_port_map = node_iter->second;
  auto ports_iter = node_peer_port_map.find(local_port->peer_port_name);
  if (ports_iter == node_peer_port_map.end())
    return;

  auto& local_ports_with_this_peer = ports_iter->second;
  local_ports_with_this_peer.erase(local_port_name);
  if (local_ports_with_this_peer.empty())
    node_peer_port_map.erase(ports_iter);
  if (node_peer_port_map.empty())
    peer_port_maps_.erase(node_iter);
}

void Node::SwapPortPeers(const PortName& port0_name,
                         Port* port0,
                         const PortName& port1_name,
                         Port* port1) {
  ports_lock_.AssertAcquired();
  port0->AssertLockAcquired();
  port1->AssertLockAcquired();

  auto& peer0_ports =
      peer_port_maps_[port0->peer_node_name][port0->peer_port_name];
  auto& peer1_ports =
      peer_port_maps_[port1->peer_node_name][port1->peer_port_name];
  peer0_ports.erase(port0_name);
  peer1_ports.erase(port1_name);
  peer0_ports.emplace(port1_name,
                      PortRef(port1_name, base::WrapRefCounted<Port>(port1)));
  peer1_ports.emplace(port0_name,
                      PortRef(port0_name, base::WrapRefCounted<Port>(port0)));

  std::swap(port0->peer_node_name, port1->peer_node_name);
  std::swap(port0->peer_port_name, port1->peer_port_name);
}

void Node::MaybeResendAckRequest(const PortRef& port_ref) {
  NodeName peer_node_name;
  ScopedEvent ack_request_event;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state != Port::kReceiving)
      return;

    if (!port->sequence_num_acknowledge_interval)
      return;

    peer_node_name = port->peer_node_name;
    ack_request_event = std::make_unique<UserMessageReadAckRequestEvent>(
        port->peer_port_name, port_ref.name(),
        port->next_control_sequence_num_to_send++,
        port->last_sequence_num_acknowledged +
            port->sequence_num_acknowledge_interval);
  }

  delegate_->ForwardEvent(peer_node_name, std::move(ack_request_event));
}

void Node::MaybeForwardAckRequest(const PortRef& port_ref) {
  NodeName peer_node_name;
  ScopedEvent ack_request_event;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state != Port::kProxying)
      return;

    if (!port->sequence_num_to_acknowledge)
      return;

    peer_node_name = port->peer_node_name;
    ack_request_event = std::make_unique<UserMessageReadAckRequestEvent>(
        port->peer_port_name, port_ref.name(),
        port->next_control_sequence_num_to_send++,
        port->sequence_num_to_acknowledge);

    port->sequence_num_to_acknowledge = 0;
  }

  delegate_->ForwardEvent(peer_node_name, std::move(ack_request_event));
}

void Node::MaybeResendAck(const PortRef& port_ref) {
  NodeName peer_node_name;
  ScopedEvent ack_event;
  {
    SinglePortLocker locker(&port_ref);
    auto* port = locker.port();
    if (port->state != Port::kReceiving)
      return;

    uint64_t last_sequence_num_read =
        port->message_queue.next_sequence_num() - 1;
    if (!port->sequence_num_to_acknowledge || !last_sequence_num_read)
      return;

    peer_node_name = port->peer_node_name;
    ack_event = std::make_unique<UserMessageReadAckEvent>(
        port->peer_port_name, port_ref.name(),
        port->next_control_sequence_num_to_send++, last_sequence_num_read);
  }

  delegate_->ForwardEvent(peer_node_name, std::move(ack_event));
}

Node::DelegateHolder::DelegateHolder(Node* node, NodeDelegate* delegate)
    : node_(node), delegate_(delegate) {
  DCHECK(node_);
}

Node::DelegateHolder::~DelegateHolder() = default;

#if DCHECK_IS_ON()
void Node::DelegateHolder::EnsureSafeDelegateAccess() const {
  PortLocker::AssertNoPortsLockedOnCurrentThread();
  base::AutoLock lock(node_->ports_lock_);
}
#endif

}  // namespace ports
}  // namespace core
}  // namespace mojo